Antisocial peer behavior has been shown to be related to a variety of negative outcomes, including low academic performance, delinquency, substance abuse, and inept social relations (Block, Block & Keyes, 1988; Dishion et al,, 1991; Kupersmidt, Coie, & Dodge, 1990; Parker & Asher, 1987). Given the negative outcome to self and society, it is clear that steps should be taken to fully understand the nature and precursors of antisocial behavior in an effort to develop preventative intervention programs. The goal of the proposed project is to study children's peer relations from a developmental framework with the view toward a future preventative intervention design. Parent-child interaction data from 150 twin pairs (75 monozygotic, 75 dizygotic) were collected when children were between 6-11 years old. A multi-method assessment of peer relations including observational data of twin-peer dyads in five situations mirroring typical school interactions took place approximately 18 months later. All data have been collected, but the peer observational data have not been coded or analyzed. The aims of this proposal are as follow: (1) to examine and develop a construct for high-risk peer behavior characterized by socialization with delinquent peers, engagement in discussion of antisocial activities, and absence of positive, reciprocal interactions; (2) to predict high- risk peer behavior from earlier parent-child interactions; (3) to assess the magnitude of sibling resemblance in antisocial peer behavior, and to disentangle the resemblance due to genetic sources versus those due to shared environmental sources; and (4) to use the results from this study to inform future research aimed at understanding and preventing children's high-risk peer behavior by (a) providing information about particular characteristics or contexts that put a child at risk for antisocial behavior, and (b) describing which of these characteristics are more influenced by environmental and genetic sources so that effective interventions can be steered toward behaviors that appear most malleable.